Lychee (Litchi chinensis Sonn.) is a subtropical evergreen belonging to the family Sapincaceae. When the lychee fruit is fully mature, its pericarp (shell) is thin, hard and somewhat warty in texture. (The term "litchee nut" is sometimes used.) Depending on the variety, the color of the pericarp is bright red or rose colored, with/without some green color. The edible portion of the fruit is a white to creamcolored translucent pulp that surrounds a glossy, brown seed.
The harvesting season for lychee fruit is very short, lasting only a few weeks, and the fruit are highly perishable. The pericarp is subject to red color loss and development of browning, whether stored at room temperature, under refrigeration, or the frozen state. Discoloration of lychee renders them less attractive to users and is accompanied by other changes which result in spoilage of the food. Loss of red color is an indication of lack of freshness. Consequently, marketing of fresh lychee fruit has been restricted. This has led to preserving lychee fruit by canning, and/or drying procedures.
When fresh lychees are stored at room temperature, dessication with its accompanying loss of red color and development of browning can occur very quickly. The appearance of browning renders the fruit unsalable and is thus of commercial importance. The attractive shelf life of fresh lychees without any protection is not over 72 hours when held at room temperature.
Browning is believed to be induced by dessication of the pericarp and the accompanying enzymatic reaction of polyphenol oxidase. See Akamine (1960), Tech. Prog. Rep. 127, HAES, Univ. of Hawaii.
In 1960 Goto reviewed procedures for the processing of lychee fruit. Goto (1960) in "Technical Papers of the First Pacific Rim Food Conf.", Honolulu, Hi., May 19-25, 1960, pp. 15-23. This review reported that tests conducted in Florida involved dipping the fresh lychee for one minute in a 1.5% aqueous solution of the sodium salt of dehydroacetic acid followed by air-drying. But this treatment kept only 43% of the treated fruit in good condition for ten days. It was also reported that freezing of lychee fruit preserved the reddish color of the pericarp while the fruit remained frozen, but that on thawing the red color was lost and the fruit rapidly turned brown.
Heretofore attempts to use chemical treatments have provided only a partial answer to the problem of prevention of pericarp browning. Treatment with ascorbic acid or citric acid solutions were found to be ineffective in preventing browning at room temperature. Akamine (1960), cited above. Kuhn had reported that dried lychee fruits which were blanched, sulfited and dipped in 0.5% citric acid prior to dehydration had improved color quality. Kuhn (1961), Progress Report, Fla. Agr. Exp. Sta., published in Journal of Florida State Horticultural Society (1962) 75:273-277. However, Ross et al. continuing the studies of Kuhn, found that this predrying treatment degraded the taste properties of the fruit. Ross et al. (1969), Trans. of the Amer. Soc. of Agric. Engn., 12(1):20-26.
Studies on the prevention of browning have been conducted in China where lychees are an important fruit crop. See Report of the Guang Dung Province Coordinating Committee on Litchi Storage (1975), Acta Botanica Sinica, 17:303-308 (in Chinese). Steaming (blanching) was tested in combination with several chemical treatments which included applying an aqueous solution of citric acid. While the pericarp color could be maintained during frozen storage, browning occurred within a few hours after thawing. In one test, the blanched fruit were treated with 5-10% aqueous citric acid, or a solution of 5% citric acid and 0.1% ascorbic acid. The frozen fruits after thawing retained their red color for only two to three hours.